Safety process for pressure equipment in contact with corrosive fluids

ABSTRACT

Method for the safety and extension of the operating life of pressure equipment having an internal chamber suitable for containing a process fluid, surrounded by a pressure-resistant body ( 1 ) equipped with weep-holes ( 2 ), made of a material subject to corrosion by contact with said process fluid operation, coated inside with an anticorrosive lining ( 4 ) made up of several elements welded to each other, wherein said lining weldings ( 3 ) are completely isolated from contact with the process fluid of the normal operating run, by a coating with adjoining strips (or plates) ( 10, 10′, 10″, 10′″ ), of the same material as said lining ( 4 ) or other corrosion-resistant material weldable thereto, which are subsequently seal-welded on the edges to said lining ( 4 ) and to each other, characterized in that the arrangement and welding of the edges of these strips ( 10, 10′, 10″, 10′″ ) are such to create a network of underlying interstices (or meati) ( 9,11 ), communicating with each other and at least one weep-hole.

The present invention relates to a method for the safety of pressureequipment in contact with corrosive fluids and to the modified equipmentthus obtained.

More specifically, the present invention relates to a method for thesafety of equipment normally operating under pressure, which is incontact with corrosive fluids and therefore comprises anticorrosivelining overlying the sealing structure (pressure-resistant body).

Typical equipment of this kind is that which is present in manyindustrial chemical plants, such as, for example, reactors,heat-exchangers, condensers and evaporators, whose operating conditionscomprise pressures of between 50 and 1000 bars and temperatures ofbetween 100 and 500° C., in contact with acid, basic or generally salinefluids having high corrosive potential especially with respect to carbonor low-alloy steel which is the material normally selected for thesealing of equipment.

Typical processes which require the use of high pressure equipment incontact with corrosive fluids are, for example, those for the productionof urea by direct synthesis starting from ammonia and carbon dioxide. Inthese processes, ammonia generally in excess and carbon dioxide arereacted in one or more reactors, at pressures usually of between 100 and250 bars and temperatures of between 150 and 240° C., obtaining amixture at the outlet consisting of a water solution of urea, ammoniumcarbamate not transformed into urea and the excess ammonia used in thesynthesis. The reaction mixture is purified of the ammonium carbamatecontained therein by its decomposition in decomposers operating, insuccession, at gradually decreasing pressures. In most of the existingprocesses, the first of these decomposers operates at pressures whichare basically equal to the synthesis pressure or slightly lower, andnormally uses “stripping” agents to decompose the ammonium carbamatewith the contemporaneous removal of the decomposition products. The“stripping” agents can be inert gases, or ammonia or carbon dioxide, ormixtures of inert gases with ammonia and/or carbon dioxide, the“stripping” also being possible by using the excess ammonia dissolved inthe mixture coming from the reactor (auto-stripping) without supplyingtherefore any external agent.

The decomposition products of ammonium carbamate (NH₃ and CO₂) togetherwith the possible “stripping” agents, excluding inert gases, arenormally condensed in suitable condensers obtaining a liquid mixturecomprising water, ammonia and ammonium carbamate, which is recycled tothe synthesis reactor. In plants which are technologically moreadvanced, at least one condensation step is carried out at pressureswhich are basically equal to or slightly lower than those of thereactor.

As a reference, it is possible to cite, among the many existing ones,patents U.S. Pat. Nos. 3,886,210, 4,314,077, 4,137,262, and publishedEuropean patent application 504.966, which describe processes for theproduction of urea with the above characteristics. A wide range ofprocesses mainly used for the production of urea is provided in“Encyclopedia of Chemical Technology”, 3° Edition (1983), Vol.23, pages548-574, John Wiley & Sons Ed.

The most critical steps in the process are those in which the ammoniumcarbamate is at its highest concentration and highest temperature, andtherefore in the above processes, these steps coincide with the reactorand subsequent equipment for the decomposition (or stripping) andcondensation of the ammonium carbamate operating under analogous orsimilar conditions to those of the reactor. The problem to be solved inthis equipment is that of the corrosion and/or erosion caused by theammonium carbamate, ammonia and carbon dioxide which act as highlycorrosive agents, especially in the presence of water, at the hightemperatures and pressures necessary for the synthesis of urea.

Various solutions to the problems of corrosion of the type describedabove have been proposed, many of which have been applied in existingindustrial plants. Numerous metals and alloys are in fact known whichare capable of resisting for sufficiently long periods, in variouscases, to potentially corrosive conditions which are created insideindustrial chemical equipment. Among these lead, titanium, zirconium,tantalium and several stainless steels such as, for example, AISI 316L(urea grade), INOX 25/22/2 Cr/Ni/Mo steel, austenitic-ferritic steels,etc., can be mentioned. However, for economic reasons, the above type ofequipment is not normally entirely constructed with thesecorrosion-resistant alloys or metals. Usually hollow bodies, containersor columns are produced in normal carbon or low-alloy steel, possiblywith several layers, having a thickness varying from 20 to 400 mm,depending on the geometry and the pressure to be sustained(pressure-resistant body), whose surface in contact with the corrosiveor erosive fluids is uniformly covered with an anticorrosive metallining from 2 to 30 mm thick.

In the above plant equipment or units, the anti-corrosive lining isproduced by the assembly and welding of numerous elements appropriatelyshaped to adhere as much as possible to the form of thepressure-resistant body, in order to create, at the end, a structurehermetically-sealed against the high operating pressure. The differentjunctions and weldings carried out for this purpose frequently requirethe use of particular techniques depending on the geometry and nature ofthe parts to be joined.

Whereas stainless steel can be welded to the underlying“pressure-resistant body” made of carbon steel, but has a higher thermalexpansion coefficient which favours, during operation, the creation offractures along the welding line, titanium cannot be welded to steel andin any case has analogous fracture problems in the weldings as it has anexpansion coefficient which is much lower than carbon steel.

For this reason resort is made to techniques which often require complexequipment and operating procedures. In certain cases the lining iseffected by welding deposit instead of plates welded to each other andonto the pressure-resistant body. In other cases, especially withmaterials which cannot be welded to each other, it is necessary to“explode” the lining onto the pressure-resistant body to be sure ofobtaining a satisfactory support.

A certain number of “weep-holes” are however applied to all the aboveequipment for the detection of possible losses of anticorrosion lining.

A weep-hole normally consists of a small pipe 5-30 mm in diameter madeof a material which is resistant to corrosion and is inserted in thepressure-resistant body until it reaches the contact point between thelatter and the lining in corrosion-resistant alloy or metal. If there isa loss of lining, owing to the high pressure, the internal fluid, whichis corrosive, immediately spreads to the interstitial area between thelining and the pressure-resistant body and, if not detected, causesrapid corrosion of the carbon steel of which the latter is made. Thepresence of weep-holes enables these losses to be detected. For thispurpose all interstitial areas underneath the anticorrosion lining mustcommunicate with at least one weep-hole. The number of weep-holes isusually from 2 to 4 for each ferrule, therefore, for example, in areactor of average dimensions, having a surface expansion of about 100m², there are normally about 20 weep-holes.

The above equipment also has, normally in the upper part, at least onecircular opening, called “man-hole”, which allows access to operatorsand equipment for inspections and minor internal repairs. These 10openings usually have diameters of between 45 and 60 cm and at the mostallow the passage of objects having a section within these dimensions.

In spite of the above measures, it is generally known that the weldinglines and points of the protective “lining” form a weak point in thestructure of chemical equipment. In fact microfractures can be foundduring operation for the above reasons of different thermal expansionbetween the materials of the pressure-resistant body and anticorrosivelining, and also preferential corrosions on the weldings or surroundingareas, owing to imperfections in the structure of the metal and todifferences in the electrochemical potential between the welded metals.A loss of protective lining therefore most probably occurs near itswelding points. On the other hand there is no possibility in practice ofapplying a monoblock lining.

As already mentioned, in the case of a loss, the fluid flows out of thelining and floods the interstices or meati or void channels presentbetween the lining and pressure-resistant body. In these cases the lossis normally detected through the weep-hole, but corrosion may occurhowever, even extensively, in the underlying carbon steel, before theloss is noticed. In the most serious cases which have led to seriouscorrosion and explosion of the equipment, the outflowing fluid, forexample a concentrated solution of ammonium carbamate in a synthesisplant of urea, can form semisolid mixtures together with the corrosionresidues, blocking the vents towards the weep-holes, thus preventing theloss itself to be detected. In the site of the loss, which can no longerbe revealed, the corrosive fluid continues its action on thepressure-resistant body, deeply corroding the structure, making itunusable, or even worse, causing the equipment to explode.

In order to avoid these phenomena, numerous solutions have beenproposed, such as, for example, in German patent DE 2.052.929, accordingto which a cover is made with a double lining interrupted bycommunication channels, thus incurring a considerable increase in theproduction costs of the equipment, and without providing a satisfactorysolution to the problem of the contact of the pressure-resistant bodywith the process fluid in the case of a possible loss.

In practice, however, most of the existing chemical plants, especiallythose not of recent construction, have a simple lining with circular andlongitudinal weldings, in which the only safety element for detectinglosses is represented by weep-holes. For the safety regulationspresently required, this solution is completely unsatisfactory and thereis a strong demand in the field for increasing both the averageoperating life and the capacity and rapidity of detecting possiblelosses (with a consequent increase in security) of the chemicalequipment in contact with corrosive substances.

The Applicant has now found a satisfactory and advantageous solution tothe above drawbacks with a simple and innovative approach which allowsan increase in the duration and reliability of pressure equipmentcomprising a pressure-resistant body consisting of a material subject tocorrosion by contact with the process fluid, and an internalanticorrosive lining in contact with said fluid, even when thisequipment is already operating in the plant. Particularly in the lattercase, the safety process can be carried out without removing theequipment from the plant and using the man-hole as the only operativeaccess to the inside of the equipment.

The present invention therefore relates to pressure equipment comprisingan internal chamber suitable for containing a process fluid, surroundedby a pressure-resistant body equipped with weep-holes, consisting of amaterial subject to corrosion by contact with said process fluid duringoperation, coated inside with an anticorrosive lining made up of severalelements welded to each other, a method for avoiding contact of saidpressure-resistant body with the process fluid as a result of a possibleloss from the weldings, and consequently increasing the safety of saidpressure equipment, comprising the following steps:

a) extension of at least a part of the weep-holes through the lining soas to form an outlet in the internal surface of the equipment;

b) covering the weldings with adjoining strips (or plates) of the samematerial as the lining or other corrosion-resistant material weldablethereto, previously shaped to suitably lay on the surface of the liningnear the weldings;

c) placing further strips of the same material as the lining, or othercorrosion-resistant material weldable thereto, each adjoining to atleast one of the above strips of step (b), until all the outlets of theweep-holes are covered;

d) hermetically welding the edges of each strip of steps (b) and (c)onto the lining and edges of other adjoining strips, to obtain, betweeneach of these strips and the underlying surface of the lining and/or itsweldings, a hermetic interstitial area (or meati) with respect to theinternal chamber and suitable for the flow of the process fluid;

characterized in that:

the arrangement and weldings between the edges of at least a part of theadjoining strips, are effected so that, beneath each of the weldingsbetween the adjoining edges, there is an opening between the existinginterstitial areas (or meati) on each side of the welding, theseopenings being hermetic with respect to the internal chamber and in sucha quantity and so arranged as to put each interstitial area (or meatus),or part of it, in communication with at least one of the weep-holeoutlets.

According to the above method, the different overlying elements are soarranged as to form the internal wall of the equipment, so that, in caseof a loss at a point near the welding in contact with the process fluid,the fluid itself, before reaching one of the weep-holes appropriatelyextended towards the lining (normally corresponding to those alreadyexisting before the safety intervention), enters in contact only withthe surfaces of a corrosion-resistant material, thus avoiding anypossible damage of the pressure-resistant body. At the same time, thearrangement of the different parts inside the equipment and the presenceof meati passing between the weldings of two adjoining covering strips,ensures the rapid detection of the fluid flowing out of a possible loss,using the same weep-holes existing before the intervention of thepresent invention. It is therefore possible to rapidly detect a possibleloss during operation from a welding of the lining complex, and at thesame time maintaining the integrity of the pre-existing structure as itis not normally necessary to apply other weep-holes, and avoiding anycontact of the pressure-resistant body with the process fluids at themoment of a possible loss.

The application, during the embodiment of the method, of one or moreweep-holes in addition to those already existing is not, however,excluded from the scope of the present invention, especially whenparticular geometries and arrangements of the elements make it necessary(for example near the outlets), provided the number is limited, normallyless than 30%, preferably less than 10% than the original ones.

A further object of the present invention relates to equipment obtainedby the embodiment of the above method. In this equipment the originalweldings of the lining are not in contact with the process fluid duringoperation, as they are covered, hermetically, by the above strips (orplates) of corrosion-resistant material. The risk is thus avoided of aprolonged action of the process fluid on these weldings causing theirperforation, by local corrosion or erosion, with the consequentdisastrous effects of an outflow of the fluid in direct contact with theeasily corrodible material of the pressure-resistant body. In the caseof a possible loss of one of the weldings subsequently effected on theedges of the covering strips to ensure the hermetic sealing of theunderlying interstices (or meati), the process fluid is directed intothese until it reaches the nearest outlet of a weep-hole, but it has nocorrosive effect, at least in the relatively rapid times necessary fordetecting the loss, on the surfaces of the materials with which it is incontact, as these materials, in accordance with the present invention,are all resistant to corrosion.

As previously specified, the method of the present invention can beparticularly applied to the high or medium pressure section of a plantfor the synthesis of urea. These can be basically identified insynthesis reactors of urea, equipment for the decomposition ofnon-transformed carbamate and containers for the condensation of NH₃ andCO₂ with the formation of carbamate solutions.

The term “adjoining strips (or plates)” as used in the present inventionand claims, refers to two or more strips, each of which has at least apart of the edge adjacent to or in contact with at least a part of theother. The term “adjoining edges” refers to these edges of stripsadjoining, adjacent to or in contact with each other.

The term “communication”, as used in the present description and in theclaims, should be considered as referring to the behaviour of a fluid,for which two points (or areas) are communicating if a fluid,particularly the process fluid, can flow from one to the other. The term“original”, as used hereafter with reference to the elements ofequipment such as weldings, lining, weep-holes, etc., identifies thoseelements already present in the equipment before the application of themethod of the present invention.

The equipment to which the method of the present invention is applicablecan be any known pressure equipment in contact with potentiallycorrosive fluids during operation. This equipment normally comprises asteel pressure-resistant body capable of resisting even very highoperating pressures (up to 1000 bars and over, preferably between 100and 500 bars), but subject to corrosion if placed directly in contactwith process fluids. This, depending on project requirements, can haveseveral layers or a single wall, possibly subjected to annealing. In theinternal chamber, in contact with the process fluid there is a lining ina corrosion-resistant material, which is usually a metal selected fromstainless steel, special austenitic-ferritic steels, lead, titanium,zirconium, vanadium, tantalium or one of their alloys. The lining can bewelded to the pressure-resistant body, or, in many cases, just fittedonto it. The lining is produced, according to the known art, by weldingplates (or ferrules) of the metal selected to each other, until theinternal surface of the pressure-resistant body is completely covered,as well as the parts inside the outlets and man-hole which normally formpart of the equipment. The weldings of the lining are normally fittedonto strips of the same material as the lining, preferably inserted intoa groove mechanically applied to the pressure-resistant body. Aspreviously mentioned, there are numerous weep-holes in thepressure-resistant body, for the purpose of controlling possible lossesof lining during operation. A detail of the arrangement of the elementsin equipment of the type specified above is schematically represented inFIG. 1 enclosed, relating to a section comprising a welding of thelining and a weep-hole.

According to the method of the present invention, in step (a) at least apart of the existing weep-holes are extended towards the lining, bydrilling or any other known technique, until it reaches the internalsurface. Each weep-hole comprises an internal lining of anti-corrosivematerial, which is also extended and welded onto the edges around theoutlet thus produced. Each outlet thus forms an opening in the lining,preferably having a diameter of between 5 and 30 mm. It is not necessaryto extend all the existing weep-holes, but only a sufficient number toguarantee easy communication with all the interstitial areas (or meati)produced in the subsequent steps of the present method. The number ofweep-holes actually extended can be evaluated by the expert in thefield, and is normally between 70 and 100% of those existing, dependingon the dimensions and geometry of the equipment and the surface densityof the holes themselves.

The application, during the embodiment of the method, of one or moreweep-holes in addition to those already existing is not, however,excluded from the scope of the present invention, especially whenparticular geometries and arrangements of the elements make it necessary(for example near the outlets, provided the number is limited, normallyless than 30%, preferably less than 10% than the original ones.

In step (b) of the method of the present invention, the weldings of thelining are covered by suitably shaped strips (or plates), resistant tocorrosion under the operating conditions of the equipment. In most casesand particularly in plants for the production of urea, the chemicalequipment has cylindrical, or curved sections. The above strips shouldtherefore be appropriately curved or shaped to adapt themselves to thesurface to be covered. However as they are easily deformed, the suitablecurvature can be obtained with normal instruments available to expertsin the field.

The strips are arranged adjacently one after the other on all theweldings so as to form, after application, a regular surface withoutgaps. It is preferable to use strips having a width of between 50 and300 mm, and a length varying from a few centimetres to several meters,depending on the requirements. The length and shape of the stripshowever are preferably selected to as allow easy access inside theequipment through the man-hole. Strip thicknesses of between 2 and 30 mmare preferably used, selected on the basis of the potential corrosiveand/or erosive action of the process fluid.

Two adjoining strips can be arranged in various ways according to thepresent invention, provided this allows: a hermetic welding system ofthe edges of the strips, which isolates the underlying weldings of thelining from the process fluid during normal operation (according to step(d) below), and suitable communication for the flow of a fluid betweenthe interstitial areas present under each of the two adjoining strips.The strips will normally be consecutive, i.e. joined one after anotherby the transversal edges, or strips perpendicular to each other, inwhich a transversal edge is joined to a longitudinal edge (parallel tothe covered welding). In the junctions between two adjoining strips,different measures can be carried out, all included in the scope of thepresent invention. It is possible, for example, to put a short part ofthe edge of one of the strips over the edge of the other, giving theformer an “S” shape. Or the two adjoining edges can be placed next toeach other; or again, a metal plate can be placed under two adjoiningedges adjacent in the junction area, possibly forming a cavity in theunderlying lining (and welding), suitable for containing a plate, toimprove the support of these adjoining edges.

According to the present invention, the covering strips consist of thesame metal as the original lining, or a metal or alloy weldable thereto.This can be selected each time from materials known to becorrosion-resistant under the operating conditions of the equipment.This metal or metal alloy is preferably selected from titanium,zirconium, or their alloys, or particularly, from stainless steels suchas, for example, AISI 316L steel (urea grade), INOX 25/22/2 Cr/Ni/Mosteel, special austenitic-ferritic steels, etc. The selection of a metalwhich has a higher resistance to corrosion (however measured) than thatof the original lining is left to the expert in the field.

The covering strips of the weldings can be fixed, before being welded inturn, with the normal methods available to experts in field, providedthese are compatible with the operating conditions of the equipment.Mechanical fixings or welding points can normally be used.

Before covering the weldings of the lining according to step (b), it ispreferable, according to the present invention, to mechanically treatthe surfaces of the weldings and lining on which the above strips are tobe placed, for example by grinding, to clean them and make them moreuniform and without defects.

Step (c) of the present method is basically carried out analogously tostep (b) above, with the difference that each strip (or plate) is notintended in this case to cover a welding of the lining, but ispositioned on the surface of the lining, adjacent to at least one of thecovering strips placed in accordance with step (b), and in the directionof at least one of the weep-holes, until the outlet on the surface ofthe lining itself is completely covered. In this way, by welding theedges according to the subsequent step (d), interstitial areas areformed communicating with this outlet and, directly or indirectly, withat least some of the interstitial areas formed near the originalweldings of the lining. According to the method of the presentinvention, all the outlets of the weep-holes are covered with strips asdescribed above, forming, by means of the underlying interstitial areas(or meati), obtained after the welding of step (d), intercommunicatingpassages from each point of the original weldings of the lining to atleast one outlet of a weep-hole.

If one or more of the weep-hole outlets is applied through one of theoriginal weldings of the lining, it is up to the operator to cover theoutlet with the same strips used for covering the weldings, obviouslywithout using any further strip according to step (c).

Also in step (c), it can be advantageous to carry out the differentoperations similarly to step (b). In particular, for example, to grindthe supporting area of the strip to clean it and make it more uniformand without defects.

According to a preferred embodiment of the present method, in steps (b)and (c), a groove is produced in the surface of the lining or itsweldings, underneath the covering strips. This groove normally has awidth of between 5 and 20 mm, a depth of between 1 and 5 mm, selected onthe basis of the thickness of the lining and the rheological propertiesof the process fluid. In particular, according to the present invention,the depth of this groove is preferably less than 30% of the thickness ofthe original lining.

This groove is preferably applied along all the original weldings of thelining, and in its surface when there is no welding, as in the case ofthe strips arranged in accordance with step (c). The groove has thefunction of facilitating the flow of the fluid coming from a possibleloss of the weldings along the edges of the strips, making the detectionof the loss more rapid and secure. The role of the groove near thejunctions between two adjoining strips (or plates) is particularlyadvantageous.

Step (d) of the method of the present invention comprises the welding ofthe edges of the strips (or plates) shaped and arranged as described insteps (b) and (c). The welding method is not critical and any of themethods available in the known art can be used, provided it guaranteesthe production of corrosion-resistant weldings and mechanical propertiessuitable for the operating conditions of the equipment.

The welding is preferably carried out with arc electrodes or “T.I.G.”with wire rods. The longitudinal edges are welded onto the surface ofthe underlying lining, and the adjoining edges of each pair of strips toeach other. The latter can at the same time also be welded to theunderlying lining. In this way, underneath, between the surface of eachstrip (or plate) and the surface of the lining near the originalwelding, there is an interstitial area (or meatus) suitable for the flowof a fluid during a possible loss.

According to the present invention, the welding of at least a part ofthe adjoining edges of the strips is carried out so that there remainsan opening underneath the welding itself, so as to put the interstitialareas (or meati) in communication with the possible grooves existingunder the strips by each side of the welding. This opening, or passage,under the welding between adjoining edges, must be hermetic in everypoint with respect to the internal chamber of the equipment, where theprocess fluid is present during normal operation.

According to the present invention the appearance and arrangement ofthese intercommunicating openings are not critical, provided they complywith the above demands and the arrangement is such that the openings, asa whole, in the case of a loss from the weldings of the strip edges,allow the process fluid to flow from any point of the above interstitialareas (or meati), until it reaches at least one of the weep-holeoutlets. It is not necessary however for all the interstitial spaces (orareas) to be intercommunicating, as it is sufficient that there becommunication, directly or indirectly through a sequence of openings andinterstitial areas, with at least one of the weep-hole outlets. It ispreferable, according to the present invention, for only from 50 to 80%of the weldings between adjoining edges to comprise an underlyingintercommunication opening.

Depending on the way in which the adjoining strips and edges arearranged in steps (b) or (c), there are various solutions for thepractical embodiment of the invention.

For example, if the adjoining edges of two strips have been partiallysuperimposed (as schematically illustrated in FIGS. 4 and 6 enclosed),it is normally sufficient to weld all the external edges of the stripsthemselves to the underlying lining and to each other. The edge of theunderlying strip, in the super-imposition area, remains on the insideand is not therefore welded, preventing the welding deposit from locallyblocking the interstice (or groove) and thus ensuring the presence of anintercommunication opening.

According to another form of embodiment, in steps (b) and/or (c) (asalready mentioned), a flat plate of the same material as the strips isplaced under the junction between two adjoining strips, preferably in acavity especially prepared in the original welding and/or lining, andthe adjoining edges of these are placed over this, adjacent to eachother. This kind of arrangement of the elements corresponds to what isschematically illustrated in FIG. 3. The flat plate has a width andlength which are such as to completely be completely covered by thestrips, and a thickness normally of about 2-4 mm. The edges of thestrips are then hermetically welded to each other (where adjoining) andto the underlying lining. The flat plate under the adjoining edgesprevents a welding deposit from blocking the underlying interstitialarea (or meatus or groove).

In a further embodiment, particularly preferred, two adjoining edges areplaced adjacent to each other and only partly welded, leaving at least apart in the central area of the junction unwelded. This unwelded part,which forms a communicating opening between the interstitial spacesunder each strip at the sides of the welding, is preferably between 5and 30 mm long.

The unwelded parts are then covered by placing metal plates over them,suitable shaped and of the same anti-corrosive material as the stripsand then hermetically welding the edges of these onto the underlyingmetal. This operation must be carried out in such a way as to guaranteethe hermetic sealing of the total surface exposed to the process fluidsof the equipment. Flat plates which are suitable for this embodiment ofthe present invention have adequate dimensions for covering the entirelength of the interrupted parts and are preferably square orrectangular. The dimensions are preferably between 20 and 200 mm. Thethickness of the plates is preferably between 4 and 25 mm.

This latter embodiment of the present invention enables an arrangementof the essential elements to be obtained corresponding to thatschematically illustrated in FIGS. 2 and 5.

Other forms of embodiment, such as, for example, those previouslydescribed in particular, in the application of the method to a singlepiece of equipment, are not excluded however from the scope of thepresent invention.

In the preferred case in which grooves are applied before the placingand welding of the strips as described above, these grooves, passingunder the weldings between adjoining edges, form in themselves excellentcommunication openings.

According to a particular embodiment of the present invention, steps(a), (b), (c) and (d) can be carried out contemporaneously, in the sensethat each of the above steps can operate independently in differentareas of the equipment. For example, it can be advantageous, especiallyin equipment of large dimensions, to carry out the welding of the edgesof the strips according to step (d) in a certain area in which theoriginal weldings of the lining and weep-hole outlets have already beencovered, whereas covering steps (b) and (c) are carried out in anotherarea. However, in each part of the equipment the intervention accordingto the method of the present invention is obviously carried out withstep (d) subsequent to steps (a), (b) and (c), and step (c) subsequentto step (a), whereas the operating order between steps (b) and (c) isnot particularly critical.

The method of the present invention enables safety operations to becarried out on existing equipment which is either new or alreadyoperating in a chemical plant. The scope of the present invention alsocomprises however the application of this method during the assembly andconstruction of new equipment to improve its duration and safety.

One of the advantages of this method is the possibility of dimensioningthe strips and flat plates and suitably shaping them so that they can beinserted through the single opening of the man-hole normally existing ineach equipment. This can also involve the use of relatively smallplates, sometimes with a length of a few tens of centimetres, but thisdoes not jeopardize reaching the desired safety measures as, accordingto the present invention, no interstitial area produced under them afterweldining, however small it may be, remains isolated from at least oneweep-hole. At the end of the intervention of the present method, theprotection of the original weldings of the lining is thus guaranteedtogether with the rapid and safe detection of a possible loss duringoperation, from any point of the covering strips and weldings thereonand without any necessity of applying new weep-holes with respect to theoriginal ones, or in any case, in particular cases, applying only aninsignificant number compared to the total amount.

In addition, the method of the present invention can be carried out, forthe same reasons mentioned above, without removing any part of theequipment and without removing this from the operating site. Theapplication and completion of the method are normally possible in factwithin a week and can be carried out during a normal stoppage of theplant (also called shutdown) for its control.

The applicative characteristics of the method of the present inventionare better illustrated by referring to the drawings and diagrams shownin the enclosed figures, wherein:

FIG. 1 schematically represents a sectional view of a wall ofconventional equipment in contact with corrosive process fluids, forexample a reactor for the synthesis of urea;

FIG. 2 schematically represents a front view of a part (internal side)of the longitudinal section of equipment to which the safety method ofthe present invention has been applied;

FIG. 3 schematically represents a detail (front view and longitudinaland transversal sections) of a part of the lining welding, afterpositioning the covering plate of the present invention, comprising ajunction and welding between two adjacent parts of the flat plate;

FIG. 4 schematically represents a detail (front view and longitudinaland transversal sections) analogous to that of FIG. 3, wherein thejunction between two parts of flat plate is according to a secondembodiment of the present invention;

FIG. 5 schematically represents a detail (front view and section) of apiece of lining welding, after the safety intervention of the presentinvention, comprising the derivation point and junction with aweep-hole;

FIG. 6 schematically represents a detail (front view and section)analogous to that of FIG. 5, wherein the derivation and junction withthe weep-hole are in accordance with a different embodiment of thepresent invention.

In the figures, corresponding parts have, for the sake of simplicity,identical reference numbers. In addition the different elements are notrepresented in scale with each other to provide a better illustration ofthe distinctive characteristics of the present invention. The differentfigures enclosed are illustrative of the present invention but do notlimit its scope in any way.

The section represented in FIG. 1 shows the pressure-resistant body 1,normally made of common carbon steel, and the original lining 4 of thereactor, made of a corrosion-resistant material, which has a weldingline 3, overlapping a flat plate or strip 7 of the same material as thelining, to avoid the welding itself being in direct contact with thesteel of the pressure-resistant body. In contact with the surfacebeneath the lining is the weep-hole 2, comprising an internal lining 8,which communicates with the interstitial area created between the liningitself and the pressure-resistant body, represented by the line 5. Apossible loss from the welding 3 follows the direction 6 indicated bythe dashed line.

FIG. 2 shows again the pressure-resistant body 1, the original lining ofthe reactor 4 and the weldings 3 with the underlying flat plates 7. Thecommunication grooves 9 and 11 are also schematically represented,applied respectively on the weldings of the pre-existing lining andalong the communication lines with the existing weep-holes 2 extendedthrough the lining itself. Above the grooves are the covering strips 10,welded in turn by the edges to the underlying lining, and extending asfar as the weep-holes. In the parts 13 where two adjoining strips meetand are welded, are the flat plates 12 welded above the former strips,hermetically covering the non-welded parts 17, forming the communicationopenings between the grooves. It is also possible to see the junction 20between two adjoining strips, completely welded and without acommunication opening, which was not necessary as both the sides of thewelding already communicated with at least one weep-hole.

FIG. 3 shows the front view (A) and longitudinal (B) and transveral (C)sectional views respectively along the lines Z1 and Z2. The elementscorresponding to those already indicated in FIG. 2 have the samereference numbers. The welding detail 13 between two covering strips 10,which are adjoining, shows the groove 9 and the flat plate 14 underneaththe welding 13, which is of the same material as the lining or of adifferent material provided this is corrosion-resistant and weldable tothe lining. The function of the flat plate 14 is to prevent, at themoment of welding 13, the groove 9 from being filled with the weldingdeposit and the communication between the interstices beneath the twoadjoining strips from being interrupted. To facilitate vision, the flatplate 7 beneath the welding 3 is not indicated in view (A).

FIG. 4 shows a front view (A) and longitudinal (B) and transversal (C)sectional views respectively along lines Z3 and Z4. The elementscorresponding to those already indicated in FIG. 2 have the samereference number. The detail of the superimposition area 15 between twoadjoining covering strips 10′ and 10″, shows the underlying groove 9,which makes the interstitial spaces or meati existing between thesestrips and the lining 4 intercommunicating. The weldings around thesuperimposition area make the interstitial spaces and the groovehermetic with respect to the process fluids. This arrangement preventsthe transversal welding 16 in particular, applied between superimposedstrips 10′ and 10″ from blocking the groove 9. Also in this case, as inFIG. 3, the flat plate 7 beneath the welding 3 is not shown in view (A).

FIG. 5 schematically represents a front view (A) and a section (B),along the line Z5, of an embodiment of the junction between twoperpendicular adjoining strips, one of which is positioned to cover oneof the weep-hole outlets. In particular strip 10 can be distinguished,which covers a groove 9 applied on a welding 3 of the lining 4. Near theweep-hole 2, there is a groove 11, in the lining, which joins 9. Theflat plate 10″′ welded by the edges to the underlying lining and weldedto strip 10 along the joining line 13, is superimposed on the flat plate10″′. In the central area of the joining line 13 there is an unweldedpart 17 to ensure communication between the underlying grooves 9 and 11.This part 17 is in turn covered by the flat plate 12 whose edges arewelded to the underlying strips 10 and 10″′ to ensure hermetic sealingtowards the process fluid.

FIG. 6 schematically represents a front view (A) and a section (B) alongthe line Z6 of a detail analogous to that of the previous FIG. 5, but inwhich the communication passage between grooves and interstitial spacesis different and in some aspects is analogous to the solution describedin FIG. 4, which is however included in the scope of the presentinvention. In particular strip 10 can be distinguished, which covers agroove 9 applied on a welding 3 of the lining 4. Near the weep-hole 2,is groove 11, in the lining, which joins 9. The flat plate 10″′, issuperimposed on the groove 11, which is welded by the edges to theunderlying lining, and superimposed on strip 10 starting from thejoining line 13. The detail of the superimposition area 18 between thetwo covering strips 10 and 10″′, shows that the underlying groove 11 isnever in contact with weldings, particularly with welding 19, thusavoiding any possibility of blockage during the weldings, which arenecessary for ensuring the hermetic sealing of the system towards theprocess fluid.

A further object of the present invention relates to pressure equipmentwith an improved degree of safety, which can be obtained with the methodpreviously described, comprising an internal chamber suitable forcontaining process fluid, surrounded by a pressure-resistant bodyequipped with weep-holes, made of a material subject to corrosion bycontact with said process fluid during operation, lined, internally,with an anti-corrosive lining consisting of several elements joined toeach other by weldings, wherein, in said equipment, at least a part ofthe weep-holes is extended towards said lining until it reaches theinternal chamber, and wherein said weldings of the lining and weep-holeoutlets are completely covered with adjoining strips (or flat plates),of the same material as said lining or other corrosion-resistantmaterial weldable thereto, which are seal-welded on the edges to saidlining and to each other to avoid contact of said lining weldings andoutlets with the process fluid during normal operation, and they form,in the underlying area, interstitial areas (or meati) which are hermeticwith respect to the internal chamber, characterized in that thearrangement and weldings between the edges of at least a part of theadjoining strips, are effected so that, beneath each of the weldingsbetween the adjoining edges, there is an opening between the existinginterstitial areas (or meati) on each side of the welding, theseopenings being hermetic with respect to the internal chamber and in sucha quantity and so arranged as to put each interstitial area (or meatus),or part of it, in communication with at least one of the weep-holeoutlets.

Particular embodiments of the above equipment, which do not limit thescope of the present invention, comprise the particular arrangements ofelements schematically shown in FIGS. 2 to 6 described above.

Following the above description of the present invention in its generalcharacteristics and details, a practical applicative example is providedwhich should not be considered, however, as limiting the scope of theinvention itself.

EXAMPLE

An intervention was carried out according to the method of the presentinvention, by isolation from the process fluid and application of asafety process of the weldings of the lining of a reactor of a plant forthe production of 400 tons/day of urea.

This reactor operated at 160 bars and 190° C., with a reaction mixturecomprising, under steady operating conditions, NH₃, CO₂, urea, water andair as passivating agent. The reactor basically comprised a verticalVessel consisting of a cylindrical pressure-resistant body with a singlewall (annealed, with a thickness of about 65 mm), having an internaldiameter of 1.4 m and a length of 24 m, equipped with two forgedhemispherical caps, of about the same thickness, placed at the upper andlower ends. On the upper end there was a circular man-hole, with adiameter of about 500 mm. The internal anticorrosive lining was made ofASIS 316L steel, urea grade, and consisted, in the central area of thereactor, of semicylindrical elements welded to each other, havingaverage dimensions of 2.2×5.0 m and a thickness of about 10 mm. Near theoutlets, caps and man-hole, the lining consisted of elements of smallerdimensions and with a more complex geometry. The surface expansion ofthe internal chamber of the reactor was about 110 m². In thepressure-resistance body there were a total of 20 weep-holes, eachhaving a diameter of 20 mm, at an appropriate distance from each other.FIG. 1, described above, schematically represents a detail of thearrangement of the elements of this reactor, around a weep-hole near awelding of anticorrosive lining.

After testing the wholeness of the pressure-resistant body and ensuringthat the weldings of the lining had no defects or losses, 15 of theexisting weep-holes were extended through the lining until they reachedthe surface of the internal chamber, making sure the edges of each holeapplied were welded to the lining itself, to avoid, in the case of aloss, infiltrations of the process fluid corroding the steel of thepressure-resistant body.

The supporting surface of the covering strips (or flat plates were thenprepared by grinding both sides of the weldings of the lining. The sameoperation was carried out along the joining lines, previously marked onthe surface of the lining, between the weep-hole outlets and at leastone of the bordering weldings.

Intercommunicating grooves having a depth of about 1-1.5 mm, were thenapplied on the weldings, comprising those in the caps and around theoutlets and man-hole, as well as on the joining lines as far as theweep-hole outlets. They were subsequently covered with plates made of25/22/2Cr/Ni/Mo steel, having a width of about 100 mm and a thickness of5 mm, adequately performed and adapted by pressure to the shape of theexisting lining. The covering flat plates, most of which had a length ofbetween 1 and 3 m, were contiguously arranged so as to completely coverall the grooves applied on the surface of the lining and the weep-holeoutlets. To do this, the adjoining edges were adjacently arranged incontact with each other but without superimposing them. The edges of theflat plates were then seal-welded by electric arc to the underlyinglining and to each other if adjoining, making sure, during the weldingof the adjoining edges to each other, that an unwelded part having alength of about 20 mm was left, in the central area, in approximatecorrespondence with the underlying groove.

Some of the adjoining edges, however, were completely welded to eachother and to the underlying lining, when no communication between theunderlying grooves was necessary as each one already individuallycommunicated with at least one weep-hole. This method of procedure,although optional, enables the network of grooves applied to the liningto be divided into a limited number of areas isolated from each other(in the example, 4-5 areas), each communicating with 2-4 weep-holes.

A plate of the same material as the flat plates, square-shaped and witha side of about 40-50 mm, was then placed on top of each of theseunwelded parts to cover it completely. The thickness was about 5 mm. Theedges of each plate were then seal-welded, onto the underlying adjoiningplates.

At the end of the intervention, each of the grooves beneath the coveringflat plates generally communicated with two or three weep-holes, withoutthere being any necessity of applying any further weep-holes, withrespect to those originally existing in the pressure-resistant body. Theinside of the reactor thus modified (central area) corresponds to thediagram represented in FIG. 2, which indicates in particular the flatplates 10 placed over the grooves 9 and 11 applied respectively on theweldings 3 of the lining 4 and on the lining itself to allowcommunication with the weep-hole outlets 2. The adjacent edges of eachpair of adjoining flat plates are only partially welded to each otheralong the joining lines 13, whereas the central part 17 is not weldedand is covered, hermetically, by the plates 12. The edges 20, of a pairof adjoining flat plates, perpendicular to each other, are on the otherhand completely welded to each other, without any communication betweenthe grooves underneath each flat plate, as these are alreadycommunicating with at least one weep-hole.

FIG. 5 schematically shows a significant detail of the appearance of thereactor obtained according to the present invention, in the embodimentillustrated, relating to the assembly of the various elements in thecommunication area between a groove 9 applied on a welding 3 and theweep-hole 2, through the groove 11. In particular it is possible to seethe partial welding of the adjoining plates 10 and 10″′, and theinterrupting part 17, covered by the plate 12.

At the end of the intervention the reactor was subjected to the usualtests to ensure it functioning. In particular the following tests werecarried out:

Control of the welding with penetrating liquids according to “ASME VIII,div. 1, appendix 8”;

Gas seal test according to “ASME V, article 10”, carried out withhelium;

Pressure seal test, carried out by bringing the internal pressure of thereactor to the value specified by the project specifications (200 bars).

All of the above tests gave satisfactory results.

What is claimed is:
 1. A pressure container with an improved safetydegree and life, comprising: a pressure-resistant body equipped withweep-holes and formed of a material subject to corrosion when contactedby a process liquid; an anti-corrosive lining made up of severalelements welded to each other so as to line the inside of the pressureresistant body and define a chamber for containing the process fluid,and having a plurality of lining holes each corresponding to a weep holeforming outlets for a chamber defined by the lining; a plurality offirst strips made of a corrosion-resistant material, pre-shaped tosuitably lay on the surface of the lining near weldings of said elementsand covering the weldings; a plurality of second strips ofcorrosion-resistant material each adjoining a first strip with an outletsuch that all the outlets are covered; a plurality of strip weldsjoining the edges of said first and second strips onto the lining andadjoining strips, wherein each of said first and second strips arewelded so as to form a hermetic channel with respect to the internalchamber and suitable for the flow of the process fluid, each channelbeing in communication with at least one of the outlets.
 2. A pressurecontainer according to claim 1, wherein said pressure container isconfigured to produce urea.
 3. The pressure container according to claim2, wherein said pressure container configured to produce urea comprisesa reactor for one of a synthesis of urea, a condenser of carbamate, anda decomposition of carbamate.
 4. The pressure container according toclaim 1, wherein said pressure container is configured to withstand apressure between 100 and 500 bars.
 5. Method for increasing the safetyof a pressure container comprising an internal chamber suitable forcontaining a process fluid, surrounded by a pressure-resistant bodyendowed with weep-holes and made of a material subject to corrosion bycontact with said process fluid during running operation, said chambercoated inside with an anticorrosive lining made up of several elementswelded to each other, by avoiding contact of said pressure-resistantbody with the process fluid as a result of a possible loss from theweldings, said method comprising the following steps: (a) extending ofat least a part of the weep-holes through the lining to form an outletin the internal surface of the container; (b) covering the weldings withadjoining strips or flat plates of the same material as the lining, orother corrosion-resistant material weldable thereto, previously shapedto suitably lay on the surface of the lining near the weldings; (c)placing on the outlets of the weep-holes further strips of the samematerial as the lining, or other corrosion-resistant material weldablethereto, each adjoining to at least one of the above strips of steps(b), until all said outlets are covered; (d) hermetically welding theedges of each strip of steps (b) and (c) onto the lining and edges ofother adjoining strips, to obtain, between each of these strips and theunderlying surface of the lining and/or its weldings, a hermeticinterstitial space with respect to the internal chamber and suitable forthe flow of the process fluid; wherein at least a part of the weldingsbetween the adjoining edges of the adjoining strips are effected so thatbeneath any such welding there is an opening between the existinginterstitial spaces on each side of the welding, said openings beinghermetic with respect to the internal chamber and in such a number andso arranged as to put each interstitial space in communication with atleast one of the weep-hole outlets.
 6. The method according to claim 5,wherein said steps (a) through (d) are performed on a pressure containerhaving a pressure-resistant body having a thickness of between 20 and400 mm and made of carbon or low-alloy steel, and an anti-corrosivelining having a thickness of between 2 and 30 mm and comprising at leastone of a metal, a metal alloy selected from titanium, zirconium, lead,vanadium, tantalium, and ASIS 316L steel (urea grade), INOX25/2212Cr/Ni/Mo steel, and special austenitic-ferritic steels.
 7. Themethod according to claim 5, wherein said steps (a) through (d) areperformed on a pressure container having a pressure-resistant bodycomprising a single-wall annealed type.
 8. The method according to claim5, wherein step (a) comprises extending between 70 and 100% of theweep-holes of container as far as the chamber.
 9. The method accordingto claim 5, wherein in steps (b) and (c) each are performed using stripshaving a width of between 50 and 300 mm, and a thickness of between 2and 30 mm.
 10. The method according to claim 5, further comprising,before steps (b) and/or (c) are performed, producing a groove along thesurface of the lining, or along the weldings thereof, in the areasubsequently covered by the first and/or second strips in order to formsaid channel.
 11. The method according to claim 5, wherein said step ofproducing a groove comprises producing a groove having a width ofbetween 5 and 20 mm, and a depth of between 1 and 5 mm.
 12. The methodaccording to claim 5, wherein said steps (b) and (c) each comprisearranging the adjoining edges of the first and strips on top of eachother.
 13. The method according to claim 5, wherein said steps (b) and(c) each comprise arranging the adjoining edges of the first and secondstrips adjacent to each other.
 14. The method according to claim 5,wherein said steps (b) and (c) each further comprise: partly weldingadjacent adjoining edges, leaving an unwelded part between two ends ofsaid strips, the unwelded part preferably having a length of between 5and 30 mm; and subsequently covering the unwelded part with a plate ofthe same material as the strips; and hermetically welding the edges ofthe plate onto the underlying metal, so as to form said channel as acommunication opening underneath each plate and adjoining edges.
 15. Themethod according to claim 5, wherein said step of covering the unweldedpart with a plate comprises covering the unwelded part with a platehaving dimensions of between 20 and 200 mm and a thickness between 4 and25 mm.
 16. A pressure container according to claim 1, wherein saidpressure-resistant body has a thickness of between 20 and 400 mm and ismade of carbon or low-alloy steel, and said anti-corrosive lining has athickness of between 2 and 30 mm and comprises one of a metal, a metalalloy selected from titanium, zirconium, lead, vanadium, tantalium, ASIS316L steel (urea grade), and INOX 25/22/2Cr/Ni/Mo steel, and a specialaustenitic-ferritic steel.
 17. A pressure container according to claim1, wherein the pressure-resistant body is of the single-wall, annealedtype.
 18. A pressure container according to claim 1, wherein the firstand second strips have a width of between 50 and 300 mm, and a thicknessof between 2 and 30 mm.
 19. A pressure container according to claim 1,further comprising a groove along the surface of the lining, or alongthe weldings thereof, in the area underlying the covering strips saidgroove forming part of said channel.
 20. A pressure container accordingto claim 1, wherein the adjoining edges of the first and second stripsare arranged adjacent to each other.
 21. A pressure container accordingto claim 20, wherein the adjacent adjoining edges are only partlywelded, leaving an unwelded part between two ends of the strips,preferably having a length of between 5 and 30 mm, the unwelded part issubsequently covered by a plate of the same material as the strips, andedges of the plate are hermetically welded onto the underlying metal, soas to form a said channel, underneath each plate and adjoining edges.22. A pressure container according to claim 21, wherein the dimensionsof the plate are between 20 and 200 mm and have a thickness of between 4and 25 mm.